The invention disclosed herein pertains generally to voice detection circuits, and more particularly to voice operated switches employing syllabic rate detection circuits.
Voice operated switches (VOX's) find a variety of applications in communication radio receivers. Used in a squelch circuit, the VOX can enable audio output from a receiver only upon the reception of voice signals so that the listener is not burdened with listening to a constant level of background noise. Voice operated switches may also have a particular utility in controlling the application of power to a transmitter, or the like, such that the transmitter is powered up only during the reception of voice signals. It is apparent that the application of power only during the useful period of a transmitter can result in substantial economical benefits.
It is well known in the art that a transmission channel can be controlled by the type of voice operated switches which detect the presence or absence of voice energy vis a vis noise energy. While this method of voice detection is simple, it is subject to false triggering due to the inability to discriminate between the presence of voice and non-voice energy components.
Another voice detection method divides the voice band into two frequency bands such that the majority of voice energy falls into a lower band. The voice signals plus noise in this lower band are then compared with the noise energy in the upper band to determine the presence or absence of a voice signal. This method of voice detection is commonly known as the two-band energy detection method.
A third method, the syllabic rate detection method, overcomes the noted discrimination problem by first detecting the composite voice and noise envelope, then passing the envelope through a syllabic rate band pass filter to define the presence or absence of syllabic rate energy.